1. Field of the Invention
The present invention is directed to a method and a ballast for starting a discharge lamp, particularly a high intensity discharge lamp (HID) such as a metal halide lamp.
2. Description of the Prior Art
HID lamps are known to be slow in reaching a stable operation of emitting a destined light output when starting the lamp at a cold state. Particularly, when the lamp is used for a vehicular headlamp or a light source for an LCD projector, it is highly desired to enable a cold start with prompt rise in the light output. To this end, Japanese Patent Laid-open Publication Nos. 4-141988 and 9-82480 propose a ballast which provides, at the start of the lamp, a run-up power greater than a normal power rating required for maintaining the operation of lamp. Then, the run-up power is made to decrease with time from a maximum power rating to the normal power rating over a transition period along a particular curve. The curve of the run-up power is derived from a single charging curve of a capacitor, as shown in FIG. 15A, and is represented as a reversal of the charging curve, as shown in FIG. 15B. Since the ballast has a fixed maximum power rating for the discharge lamp, a portion of the run-up power curve above the maximum power rating should be limited to the maximum power rating, resulting a composite curve in which the maximum power rating is maintained for an initial start time period and then decrease with time to the normal power rating. As the initial start time is required to be longer for attaining a more rapid start of the lamp as indicated by solid lines in the above figures relative to those indicated by dotted lines, both curves, i.e., the charging curve and the run-up curve are made more moderate. Thus, the run-up power decreases along a more moderate slope, applying a more amount of power to the lamp during a transition period from the start to the stable lamp operation, resulting in an overshoot of the light output, as shown in FIG. 16. In order to avoid this problem, it is desired to separately control the initial start time and the curve shape in the transition period which cannot be made in the above prior art, thereby decreasing the run-up power along a moderate slope to exclude a possibility of the overshoot in the light output.